The in vitro effects of vincristine, prednisolone and phytohemagglutinin on dividing and non-dividing cells are studied by a slide chamber method and phase microscopy. The dividing cells are obtained from permanent cultures of normal and acute leukemic blood cells and from short term cultures of transplantable mouse lymphoma. The non-dividing cells are blood lymphocytes from normal persons and from patients with chronic lymphocytic leukemia. The cells are incubated with varying dilutions of reagent and the number of viable cells are counted before and after incubation for 3 to 7 days at 37 degrees C. Viable lymphocytes are counted in the cell suspensions before and after incubation with the reagent.